1. Field of the Invention
The present invention relates to a predistortion compensation apparatus, performing distortion compensation processing in advance on a transmission signal before amplification.
2. Description of the Related Art
In recent years, high-efficient digital transmission has widely been adopted in radio communication. When multilevel phase modulation is adopted in the radio communication, it is an important technique to suppress nonlinear distortion by linearizing the amplification characteristic of a power amplifier on the transmission side, so as to reduce adjacent channel leak power.
Also, when it is intended to improve power efficiency using an amplifier having a degraded linearity, a technique for compensating nonlinear distortion caused by the degraded linearity is essentially required.
FIG. 1 shows an exemplary block diagram of transmission equipment in the conventional radio equipment. A transmission signal generator 1 outputs a digital serial data sequence. A serial-to-parallel (S/P) converter 2 then converts the digital data sequence into two series, in-phase component signals (I-signals) and quadrature component signals (Q-signals), by alternately distributing the digital data sequence on a bit-by-bit basis.
A digital-to-analog (D/A) converter 3 converts the I-signal and the Q-signal into an analog baseband signal, respectively, so as to input into a quadrature modulator 4. This quadrature modulator 4 multiplies the input I-signal and Q-signal (a baseband transmission signal) by a reference carrier wave 8, and a carrier wave phase-shifted by 90° from the reference carrier wave 8, and adds the multiplied results, thus performing orthogonal transformation, and outputs the above signal.
A frequency converter 5 mixes the quadrature modulation signal with a local oscillation signal, and converts the mixed signal into a radio frequency. A transmission power amplifier 6 performs power amplification of the radio frequency signal being output from frequency converter 5, and radiates to the air from an antenna 7.
Here, in the mobile communication using W-CDMA, etc., transmission equipment power is substantially large, as much as 10 mW to several tens of mW, and the input/output characteristic (having a distortion function f(p)) of transmission power amplifier 6 shows non-linearity, as shown by the dotted line in FIG. 2. This nonlinear characteristic produces a non-linear distortion. As shown by the solid line (b) in FIG. 3, the frequency spectrum in the vicinity of a transmission frequency f0 comes to have a raised sidelobe, shifted from the characteristic shown by the broken line (a) in FIG. 3. This produces a leak to adjacent channels, resulting in adjacent channel interference. Namely, due to the nonlinear distortion shown in FIG. 2, the leak power of the transmission wave to the adjacent frequency channels becomes large, as shown in FIG. 3.
An ACPR (adjacent channel power ratio) represents the magnitude of leak power, being defined as a ratio of leak power to adjacent channels, which corresponds to a spectrum area in the adjacent channels sandwiched between the lines B and B′ in FIG. 3, to the power in the channel of interest, which corresponds to a spectrum area between the lines A and A′. Such the leak power affects other channels as noise, and degrades the communication quality of the channel of interest. For this reason, a strict regulation has been established.
The leak power is substantially small in a linear region of, for example, a power amplifier (refer to a linear region I in FIG. 2), and is substantially large in a nonlinear region II. Accordingly, in order to obtain a high-output transmission power amplifier, the linear region I has to be widened. However, this requires an amplifier having a larger capacity than is actually needed, which causes a disadvantageous problem in both cost and size of the apparatus. To cope with this problem, it has been applied to add a distortion compensation function to radio equipment so as to compensate for the transmission power distortion.
FIG. 4 shows a block diagram of transmission equipment having a digital nonlinear distortion compensation function. A digital data group (transmission signals) transmitted from transmission signal generator 1 is converted in S/P converter 2 into two series, I-signals and Q-signals. The two signal series are then input to a distortion compensator 9, which is configured of a DSP (digital signal processor) as a preferable example.
As shown in the lower part of FIG. 4 in enlargement, distortion compensator 9 includes: a distortion compensation coefficient storage 90 for storing a distortion compensation coefficient h(pi) corresponding to the power level pi (where, i=0−1023) of a transmission signal x(t); a predistortion portion 91 for performing a distortion compensation process (predistortion) onto the transmission signal, using the distortion compensation coefficient h(pi) corresponding to the transmission signal power level; and further, a distortion compensation coefficient calculator 92 for updating a distortion compensation coefficient in distortion compensation coefficient storage 90, by comparing the transmission signal x(t) with a demodulation signal (a feedback signal) y(t) demodulated in a quadrature detector 12, which will be described later, and calculating the distortion compensation coefficient h(pi) so that the difference between the above compared values becomes zero.
The signal on which the predistortion process is performed in distortion compensator 9 is input into D/A converter 3. D/A converter 3 converts the input I-signal and Q-signal into analog baseband signals, and inputs the converted signals into quadrature modulator 4. Quadrature modulator 4 performs quadrature modulation by multiplying the input I-signal and Q-signal by a reference carrier wave 8 and a carrier wave being phase-shifted by 90° from reference carrier wave 8, respectively. Quadrature modulator 4 performs quadrature modulation by adding the multiplication result, and outputs the modulated signal.
A frequency converter 5 performs frequency conversion by mixing the quadrature modulation signal with a local oscillation signal. A transmission power amplifier 6 performs power amplification of the radio frequency signal being output from frequency converter 5, and radiates to the air from antenna 7.
A portion of the transmission signal is input to a frequency converter 11 via a directional coupler 10, and input into a quadrature detector 12 after being frequency converted by the above frequency converter 11. Quadrature detector 12 performs quadrature detection by multiplying the input signal by a reference carrier wave, and by a signal being phase-shifted by 90° from the reference carrier wave, respectively. Thus, the baseband I-signal and Q-signal on the transmission side are reproduced, and then input into an analog-to-digital (A/D) converter 13.
A/D converter 13 converts the input I-signal and Q-signal into digital signals, and inputs into distortion compensator 9. Through the adaptive signal processing, using an LMS (least-mean-square) algorithm, in distortion compensation coefficient calculator 92 of distortion compensator 9, the pre-compensated transmission signal is compared with the feedback signal being demodulated in quadrature detector 12. Then, distortion compensator 9 calculates the distortion compensation coefficient h(p1) so that the difference of the above comparison values becomes zero, and updates the above-obtained coefficient having been stored in distortion compensation coefficient storage 90. Through the repetition of the calculations above, nonlinear distortion in transmission power amplifier 6 is restrained, and adjacent channel leak power is reduced.
As a configuration of the embodiment of distortion compensator 9 shown in FIG. 4, a configuration example in case of performing distortion compensation using the adaptive LMS has been disclosed, as shown in FIG. 5, for example, in the PCT Internal Publication No. WO 03/103163.
In FIG. 5, a multiplier 15a corresponds to predistortion section 91 shown in FIG. 4, in which a transmission signal x(t) is multiplied by a distortion compensation coefficient hn-1(p). Also, a distortion device 15b having a distortion function f(p) corresponds to transmission power amplifier 6 shown in FIG. 4.
Also, a portion including frequency converter 11 in which the output signal from transmission power amplifier 15b is feedbacked, orthogonal detector 12 and A/D converter 13 shown in FIG. 4 is shown as a feedback system 15c in FIG. 5
Further, in FIG. 5, distortion compensation coefficient storage 90 shown in FIG. 4 is constituted of a look-up table (LUT) 15e. Distortion compensation coefficient calculator 92 shown in FIG. 4 for generating an update value for the distortion compensation coefficient stored in look-up table 15e is constituted of a distortion compensation coefficient calculator 16 shown in FIG. 5.
In the distortion compensation apparatus having such the configuration shown in FIG. 5, look-up table 15e stores a distortion compensation coefficient for canceling the distortion of transmission power amplifier 6, a distortion device 15b, in a two-dimensional address position corresponding to each discrete power value of the transmission signal x(t).
When the transmission signal x(t) is input, an address generator 15d calculates the power p(=x2(t)) of the transmission signal x(t), and generates an address in the direction of one dimension, for example, an address in the X-axis direction which uniquely corresponds to the calculated power p(=x2(t)) of the transmission signal x(t). At the same time, address generator 15d obtains a difference ΔP from the power P1(=x2(t−1)) of a transmission signal x(t−1) at the preceding time point (t−1) having been stored in address generator 15d. Address generator 15d then generates an address in the direction of another dimension, for example, in the Y-axis direction, which uniquely corresponds to the difference ΔP.
Accordingly, address generator 15d outputs the storage location of look-up table 15e being specified by both the address P in the X-axis direction and the address ΔP in the Y-axis direction, as specified information of a readout address (AR).
Thus, the distortion compensation coefficient hn-1(p) stored in the above readout address is read out from look-up table 15e, which is used for the distortion compensation processing in multiplier 15a. 
Meanwhile, an update value for updating a distortion compensation coefficient stored in look-up table 15e is calculated in a distortion compensation coefficient calculator 16. More specifically, distortion compensation coefficient calculator 16 includes a conjugate complex signal output portion 15f, which outputs a conjugate complex signal y*(t), and multipliers 15h-15j. A subtractor 15g outputs a difference e(t) between the transmission signal x(t) and the feedback demodulation signal y(t). Multiplier 15i multiplies the distortion compensation coefficient hn-1(p) by y*(t), and obtains an output u*(t) (=hn-1(p)y*(t)). Multiplier 15h multiplies the difference e(t) being output from subtractor 15g by u*(t). Multiplier 15j multiplies a step-size parameter μ by the output of multiplier 15h. 
Subsequently, an adder 15k adds the distortion compensation coefficient hn-1(p) and the output of multiplier 15j, i.e. μe(t)u*(t), so as to obtain an update value of look-up table 15e. This update value is stored in the write address (AW) as an address corresponding to the power p(=x2(t)) of the transmission signal, being specified by the address in the X-axis direction and the address in the Y-axis direction generated by address generator 15d. 
Additionally, the readout address (AR) and the write address (AW) explained above is the same address. However, because a calculation time, etc. is required before obtaining the update value, the readout address being delayed in a delay portion 15m is used as write address.
Each delay portion 15m, 15n, 15p adds a delay time D to the transmission signal. Here, the delay time D denotes time duration from the time the transmission signal x(t) is input to the time the feedback demodulation signal y(t) is input to subtractor 15g. This delay time D to be set in each delay portion 15m, 15n, 15p is determined so as to satisfy D=D0+D1, where D0 is the delay time in transmission power amplifier 15b, and D1 is the delay time in feedback system 15c. 
Using the above configuration, the following calculations are performed.hn(p)=hn-1(p)+μe(t)u*(t)e(t)=x(t)−y(t)y(t)=hn-1(p)x(t)f(p)u*(t)=x(t)f(p)=hn-1(p)y*(t)p=|x(t)|2 Here, x, y, f, h, u and e are complex numbers, and * denotes a conjugate complex number.
Through the above calculation processing, the distortion compensation coefficient h(p) is updated so as to minimize the differential signal e(t) between the transmission signal x(t) and the feedback demodulation signal y(t). Finally, the value converges to an optimal distortion compensation coefficient, and the distortion of transmission power amplifier 6 is compensated.
Now, FIG. 6 is a schematic diagram of the data in the real part side in distortion compensation coefficient storage 90 (refer to FIG. 4), or look-up table 15e shown in the example of FIG. 5, in which the above distortion compensation coefficient h(pi) is stored. A magnitude P is set to one axis direction out of the two dimensions, while a magnitude ΔP is set to the other axis direction. In the axis direction perpendicular to these axes, a distortion compensation coefficient value h(p) is expressed.
In FIG. 6, each peak which appears in places (as an example, a portion surrounded by a circle 100) is a portion in which a distortion compensation coefficient value h(p) becomes (or is becoming) an abnormal data (that is, h(p) having a large amplitude). The phenomenon of the above generation of the peaks is caused by the calculated update value of the distortion compensation coefficient becoming an abnormal value, when a large differential signal e(t) is produced by a largely varied feedback signal due to a large phase jitter, variation of the amplification characteristic, etc. in the analog portion including power amplifier 6.
If the predistortion processing is performed on the transmission signal, and the table update processing is performed, using such an abnormal value having the above-mentioned characteristic, the abnormal value in the table further produces an updated distortion compensation coefficient of an abnormal value. Finally, the distortion compensation coefficient diverges, bringing about an abnormal amplifier output as a result of the execution of the distortion compensation processing.